The present invention relates to semiconductor instruments and, more particularly, to a heterogeneous semiconductor structure with a composition gradient, and a method for producing same.
The invention can find extensive application in producing semiconductor lasers with an adjustable generation wavelength, and semiconductor spectrometers of high resolving power.
Such instruments, produced on the basis of the proposed heterogeneous semiconductor structure with a composition gradient, may be used in such fields as laser spectroscopy of substances for fundamental research, high-sensitivity remote checking of the composition of fumes, gas mixtures and solutions, while controlling production processes, in the analysis of exhaust gases of automobiles, laser photochemistry, and in the metrology of laser irradiation.
In addition, the heterogeneous semiconductor structure with a composition gradient in accordance with the present invention can be employed in devices for high-sensitivity remote detection of substances which pollute the environment, as well as in contactless sensors for checking chemical compositions to be used in automatic production control systems, light-emitting instruments and optoelectronic and integral optics devices.
An important application of the proposed heterogeneous semiconductor structure with a composition gradient is to high-sensitivity hydrostatic pressure gauges.
Such sensors, provided with a sensitive element on the basis of the proposed heterostructure, are marked by a broader range of measurements (of 10 to 60,000 bars), as compared to conventional types of sensors.
Novel sensors on the basis of the present invention may find extensive application in automatic control and check systems, as well as in different fields of industry and research, such as the synthesis of diamonds, pressure measurements in drill holes and explosion chambers, measurements of the pressure produced by rock beds in seismic zones, weighing of heavy loads (railroad cars, trucks, machinery), and in other fields of endeavor which call for hydrostatic pressure measurements over a broad range.
There is known a heterogeneous semiconductor structure comprising a substrate coated with a first reflecting layer, and a main semiconductor layer arranged on the first reflecting layer, said main semiconductor layer being solid solution of AB.sub.x C.sub.1-x. The solid solution is composed of binary compounds with the general formulas AB and AC, whereas x represents the molar fraction of the AB compound in the solid solution AB.sub.x C.sub.1-x, which is limited by the ratio 0.ltoreq.x.ltoreq.1.
The semiconducting compounds AB and AC comprising one common chemical element denoted by letter A and variable content components denoted by letters B and C, respectively, are selected from the following pairs of elements: A.sup.II B.sup.VI and A.sup.II C.sup.VI ; A.sup.III B.sup.V and A.sup.III C.sup.V ; B.sup.III A.sup.V and C.sup.III A.sup.V ; B.sup.II A.sup.VI and C.sub.II A.sup.VI ; B.sup.IV A.sup.VI and C.sup.IV A.sup.VI ; so that said pairs of binary compounds form the following solid solutions respectively; A.sup.II B.sub.x.sup.VI C.sub.1-x.sup.VI, A.sup.III B.sub.x.sup.V C.sub.1-x.sup.V, B.sub.x.sup.III C.sub.1-x.sup.II A.sup.VI ; B.sub.x.sup.III C.sub.1-x.sup.III A.sup.V, B.sub.x.sup.IV C.sub.1-x.sup.IV A.sup.VI, where B.sup.II, A.sup.III, A.sup.V and A.sup.VI are elements of Groups II-B, III-A, V-A and VI-B, respectively, of the Periodic Table; B.sup.II and C.sup.II are two different elements of Group II-B, B.sup.III and C.sup. III are two different elements of Group III-A; B.sup.IV and C.sup.IV are two different elements of Group IV-A; B.sup.V and C.sup.V are two different elements of Group V-A; and B.sup.VI and C.sup.VI are two different elements of Groups VI-A of the Periodic Table, and "x" varies from 0 to 1.
The composition of the main semiconductor layer changes gradually and continuously along the main axis from a maximum concentration of the AB component near the first edge of the main semiconductor layer to a maximum concentration of the AC component close to the opposite edge of the main semiconductor layer. A second reflecting layer is arranged on the outer surface of the main layer. The AB and AC components are semiconductor compounds having similar crystallographic properties. The known heterogeneous semiconductor structure is produced on the basis of solid solution of CdSe.sub.x S.sub.1-x composed of binary compounds CdSe(A.sup.II B.sup.VI) and CdS(A.sup.II C.sup.VI).
The foregoing heterogeneous structure with a composition gradient is used in the main as the working irradiating element of a semiconductor laser with optical or electron excitation and an adjustable generation wavelength.
Working elements of lasers of this type can only operate when the material of the substrate is similar in its crystallographic properties (e.g., the parameters of the crystal lattice, the thermal expansion coefficient) to the solid solution AB.sub.x C.sub.1-x of the main semiconductor layer.
As a result, the known heterogeneous semiconductor structure with a composition gradient can only be produced from an extremely limited range of semiconductor components with similar crystallographic parameters which do not differ to a great extent from the crystallographic parameters of the substrate material.
Such components are, for example, CdSe and CdS which have similar crystallographic properties. On the basis of these components there may be produced a working element of a laser operating within a range of wavelengths of 0.5 to 0.7 mu.
However, in a number of cases the formation of a heterogeneous structure for a laser irradiating in a different spectral range, or a laser with an extended generation wave adjustment range, or a working element of a laser which is supposed to be highly resistant to external effects, calls for the use of different semiconductor compounds whose crystallographic properties may be different.
The use of such components for the formation of the known type of heterostructure results in numerous imperfections of the crystal structure (dislocations, mechanical stresses, cracks) which, in the course of operation of the laser element, act upon a number of radiationless recombination centers and thereby either suppress the laser irradiation or worsen the irradiation characteristics of the laser.
While using the known heterogeneous structure, it is impossible to avoid the harmful effects of the substrate upon the main semiconductor layer which is composed of components with different crystallographic properties.
There is known a method for producing the above type of heterogeneous structure with a composition gradient, which structure is solid solution of AB.sub.x C.sub.1-x. According to this method, semiconductor components are transferred through the gas phase to the surface of the substrate which is heated to a temperature above 450.degree.. The semiconductor components are transferred from a source whose temperatures is different from that of the substrate. The components enter the gas phase so that along the main axis of the semiconductor layer being formed, there is a continuous and gradual change in the concentration of atoms of B and C from a maximum concentration of B atoms near the first edge of the main semiconductor layer being formed to a maximum concentration of C atoms close to the opposite edge of this layer.
The semiconductor elements are directed to the gas phase from a source comprising two portions, the first being of the AB component, while the second is of the AC component. The continuous and gradual alteration of the composition is ensured by partially mixing AB and AC in the gas phase; a slit mask moving along the main axis of the substrate additionally provides for a continuous change in the thickness of the main layer along said axis from maximum to minimum.
The major disadvantage of the foregoing method for producing a heterogeneous semiconductor structure with a composition gradient resides in the fact that it does not always ensure a monocrystalline structure of the main semiconductor layer; on the contrary, it often results in the formation of a polycrystal which does not meet the requirements imposed by the laser irradiation, becase the polycrystalline structure accounts for numerous defects which, in the course of operation of the laser element, act upon a number of radiationless recombination centers and thus either suppress the laser irradiation or seriously affect the radiation characteristics of the laser.
For decrystallization purposes, the polycrystalline heterostructure is subjected to tempering at a high temperature. This operation makes the process complicated and lengthy and, although affecting the out, does not rule out completely all the defects that hinder the normal work of the laser.
Besides, the known method does not make it possible to produce a monocrystalline semiconductor heterostructure with a required rate of the change in the concentration of the components along the main axis. From the portions of the source, the compounds AB and AC propagate concentrically in relation to the centers of said portions; hence, a linear change in the components' concentration is ensured only in a narrow zone of the main layer being produced, which zone is close to the projection of the axis between the centers of the source's portions; it is impossible to produce a different type of change in the gradient value, while using the above type of source.
As a result, on the basis of the known heterogeneous structure, it is impossible to produce a working element of a laser with a required uniform adjustment of the generation wavelength within a broad range of wavelengths.
The known method for producing a heterogeneous semiconductor structure with a composition gradient is also disadvantageous in that on the basis of the structure it is supposed to produce, it is impossible to provide a working element of a laser having mirror layers on the side faces of the main semiconductor layer and also having a greater radiating power, as compared to known laser elements.
Such an element can only be produced on the basis of a heterogeneous structure with a constant direction of the composition gradient over the entire area of the semiconductor layer from one edge of said layer to the other. On the other hand, the known method makes it possible, as stated above, to provide a constant direction of the gradient only in a narrow zone near to the projection upon said layer of the axis between the centers of the source's portions.
Still another important drawback of the known method under review is the fact that it is impossible to control the process of doping the main semiconductor layer along the main axis and at a perpendicular to the latter. This factor limits the sphere of application of the known structure, since the radiation characteristics of the laser cannot in this case be optimized by providing for a variable doping level along the main axis, which level should correspond to the gradual change in the composition of the main semiconductor layer.
Finally, the known method does not make it possible to control the distribution of doping impurities over the thickness of the heterostructure, i.e. in the direction perpendicular to that of the composition gradient in the main semiconductor layer. As a result, the heterostructure cannot be used for working elements of other instruments.
It is an object of the present invention to provide a heterogeneous semiconductor structure with a composition gradient, which would be marked by a substantially reduced harmful effect of the substrate upon the crystalline structure and luminescent properties of the main semiconductor layer.
Thus, it is an object of the invention to provide a heterogeneous semiconductor structure with a composition gradient, which, when used as the working element of a laser, would improve the latter's radiation characteristics, i.e. reduce the density of the threshold current required to excite laser generation, as well as would improve the efficiency of converting the excitation energy into the luminous energy of the laser.
It is another object of the present invention to expand the range of compounds to be used for the formation of the main semiconductor layer, which would make it possible to select the components of the main layer from different classes of semiconductor compounds and, as a result, produce a series of heterogeneous structures to be used as working elements of lasers and photodetectors operating within a broad range of wavelengths, from the near-ultraviolet region to the infrared region.
It is also an important object of the invention to provide a heterogeneous semiconductor structure with a composition gradient, which would make it possible to raise the radiated power of lasers.
It is still another important object of the invention to provide a heterogeneous structure with an extremely small value of the composition gradient (a few mole percent per centimeter of length along the axis), which ensures a high accuracy of selecting a desired radiation wavelength of a reference laser.
It is yet another object of the invention to provide a heterogeneous structure, wherein the law of a change in the doping impurities would be independent of the law of a change in the composition of the main semiconductor layer, i.e. a heterostructure marked by a constant doping level both throughout the surface and volume of the main semiconductor layer, which would ensure a constant radiated power over the entire range of wavelengths.
Apart from the above, it is an object of the present invention to provide a method for producing the foregoing type of heterogeneous semiconductor structure with a composition gradient, which would ensure a broad range of continuous change in the composition of the main semiconductor layer of said structure (to a few tens of mole percent).
It is a further object of the invention to provide a method for producing a heterogeneous semiconductor structure, which method would make it possible to ensure a high accuracy of maintaining a predetermined composition gradient over the surface of the heterostructure, as well as the rectilinearity of the lines of the constant composition on the surface of the heterogeneous structure.
It is also an object of the invention to provide a method for producing a heterogeneous semiconductor structure with a composition gradient, which would make it possible to control the composition of said structure both over its surface and thickness in the course of producing said structure.
It is a further object of the invention to provide a method for the production of a heterogeneous semiconductor structure, which would make it possible to control the change in the doping impurity content irrespective of the change in the composition along the main axis in the course of producing said structure.
It is an object of the invention to provide a method for producing a heterogeneous semiconductor structure with a composition gradient, which structure can be employed as a working element of a laser without being subjected to thermal treatment.
Finally, it is an object of the invention to provide a simple source for carrying out the proposed method of producing a structure possessing all the abovementioned properties.